In order to fold a blade automatically, i.e. to cause it to pass from a flight configuration to a parking configuration, a motor drives a power transmission system for driving means for folding the blade. However, if some external element, e.g. an obstacle associated with the environment such as a wall or a gust of wind, should stop the blade moving while it is being folded, the torque exerted on the transmission system will increase, thereby running the risk of severely damaging the system. It is therefore essential to provide a device for limiting this torque.
A torque limiter is known that comprises a first plate that drives a second plate via a plurality of balls, the balls being housed in orifices formed in the first and second plates. The torque limiter is disposed between an actuator and a transmission system.
If the torque exerted on the transmission system becomes too great, then the balls escape from the orifices, thereby preventing the second plate being driven by the first plate. As soon as the torque returns to an acceptable level, the balls return into their housings, and the first plate can again drive the second plate.
That torque limiter satisfies requirements. However another major difficulty needs to be taken into consideration. In the event of the motor of the automatic blade-folding device breaking down, it can become impossible to fold the blade, which can lead to major problems of shortage of space on board the ship where the helicopter is parked. Since the torque limiter cannot be used as an actuator, it is necessary to provide auxiliary means for driving the transmission system. The folding device then becomes particularly complex, given the possibility of the motor jamming, which can impede maneuvering, and given the geometrical constraints imposed by the main rotor.